This invention relates generally to oil and gas well logging tools. More particularly, this invention relates to a tool for measuring gas saturation and gas pressure of earth formations through the use of gamma rays generated by a pulsed neutron source. This invention may be used in cased holes as well as open holes.
In petroleum and hydrocarbon production, there is considerable commercial value in the recovery of gas from reservoirs. Over the course of production of gas, there is an increasing influx of water into the reservoir. This may be due to natural causes or it may be, in the case of secondary recovery operations, the result of injection of water into the reservoir. The production of gas thus leads to a decrease in gas saturation of the reservoir. In addition, due to the fact that reservoirs by their very nature comprise permeable earth formations within impermeable strata, production of gas leads to a decrease of gas pressure. The decrease of gas pressure in turn affects the flow pattern of reservoir fluids. Knowledge of the gas pressure is also very helpful in reservoir development. Knowledge of gas saturation is also important in enhanced oil recovery programs (EOR) where a gas is injected into an injection well and used to direct the flow of oil from the reservoir into a production well.
A basic methodology underlying the determination of gas saturation and/or gas pressure is that of density determination. One approach involves detection of gamma radiation produced in the formation in response to a high-energy neutron source, referred to as induced gamma ray logging. When the neutron source is pulsed, gamma rays are produced by one of two reactions. The first is inelastic scattering of fast neutrons (neutrons with energies above about one MeV or within about one order of magnitude). The second mechanism is from capture of epithermal neutrons (neutrons with energy of about one eV). The third is from capture of thermal neutrons (neutrons with energy of about 0.025 eV). The fast-neutron lifetimes are very small (a few microseconds) such that during the source pulse a mixed-energy neutron field exists. Shortly after the burst, all neutrons slow down to a thermal energy level and these thermal neutrons wander about until being captured, with a lifetime in the hundreds of microseconds. Gamma rays from inelastic scattering are produced in close proximity to the accelerator, and gamma rays from thermal capture are dispersed farther from the accelerator (up to tens of centimeters). The number of capture gamma rays is strongly influenced by the amount of hydrogen and the thermal neutron capture cross section of the formation. The number of gamma rays produced from inelastic scattering is less dependent on these quantities, and a measurement of such gamma rays is more directly related to the formation density. Use of a pulsed neutron source allows capture gamma rays to be separated from inelastic gamma rays, giving a better estimate of density.
U.S. Pat. No. 3,780,301 to Smith Jr. et al. discloses a method and apparatus for determination of gas saturation using a logging tool deployed in an open borehole. A pulsed neutron source produces pulses of neutrons with energy of about 14 MeV. A single gamma ray detector measures counts of inelastic gamma rays resulting from interaction of the neutrons with nuclei in the formation. Specifically, counts are made in energy bands corresponding to C, O, Si and Ca. By comparing the Si/Ca and C/O ratios in these regions to the Si/Ca and C/O ratios for a known water sand, the relative abundance of limestone in the low hydrogen content formations may be estimated thus distinguishing gas zones from water saturated low porosity limestone.
When the wellbore in which the tool is run is an uncased reservoir, the tool is able to contact the subterranean formation itself. However, once a well has been cased, there exists a layer of steel and concrete between the interior of the wellbore where the tool is located and the formation itself. The well casing makes it difficult for signals to pass between the tool and the reservoir and visa versa. In addition, the cement can confuse the measurement of formation properties.
Formation density measurements have traditionally been made using two gamma ray detectors. In open hole situations, density estimates ρSS and ρLS made by the near and far detectors are used to get a corrected density estimate using the spine and rib method which may be represented by the equationρ−ρLS=Δρ=ƒ(ρLS−ρSS)   (1),where ƒ(·) is a function that is nonlinear, depends upon the standoff of the tool or the amount of mud cake between the tool and formation, and determined by a calibration process. This dual detector arrangement is able to compensate for standoff (in MWD applications) and mudcake thickness (in wireline applications). When used with a pulsed neutron source, correction also has to be made for variations in the source intensity, so that a two detector arrangement only gives a single estimate of density based on, for example, a ratio of the outputs of the two detectors.
For measurements made in cased holes, as noted above, there is an additional complication due to the presence of casing and cement. In order to probe the formation, neutrons must exit the tool, pass through the casing and cement and scatter, or be captured in the formation before the resulting gamma rays pass passing back through the cement and the casing to finally reenter the tool to be detected. Thus, instead of just a mudcake correction (for open hole wireline) or a standoff correction (for MWD), a cased hole density tool must be able to correct or compensate for the cement and casing, an effect which is greater than that of the mudcake. U.S. Pat. No. 5,525,797 to Moake discloses the use of a three detector tool using a chemical gamma ray source which corrects for the effects of casing. A drawback of the Moake device is the need for a relatively high energy chemical source (a safety issue) and the fact that gamma ray energies are measured (instead of count rates). In addition, it is not possible to separate inelastic gamma rays from capture gamma rays.
U.S. Pat. No. 5,825,024 to Badruzzaman discloses an apparatus for measuring the density of a subterranean formation from within a wellbore, especially a cased wellbore. The apparatus has an energy source configured to generate 14 MeV neutrons in pulses of 20 microseconds or less. The apparatus has at least three detectors for detecting gamma rays which are produced as a result of the neutron pulse. The detectors and energy source are aligned along a central axis with the energy source being at one end. Shielding is disposed between each of the three detectors and between the end detector and the energy source adjacent to it. The detectors are configured to measure gamma rays below 700 KeV and generate a signal in response thereto. The signals may then be compared to predetermined characteristic signals or computer simulations to determine the density, and hence porosity, of the formation.
Badruzzaman et al. (SPE89884) discuss the use of a four sensor arrangement for through-casing density measurements with a pulsed neutron source. These included a pseudo-density determination, C/O measurements for oil saturation determination, and pulsed neutron capture (PNC) cross section measurements for water and steam saturation.
None of the prior art recognizes the inter-relation between the effects of gas saturation and gas pressure on the measurements made by a nuclear logging tool. The present invention recognizes the inter-relation and represents a comprehensive approach to the determination of reservoir characteristics through casing.